When landing an aerial vehicle capable of VTOL (Vertical Take Off/Landing), such as an unmanned helicopter, it is important that the aerial vehicle has access to accurate information regarding movement, position and attitude relative to the ground or landing area. In particular, when trying to land on a platform of a ship, the landing process is more difficult since the sea continuously changes the attitude of the ship and the landing platform. Also the fact that the ship is moving in a specific bearing further complicates the landing on the platform. The inclination of the platform is determined by the pitch and the roll, and there is also a movement in the vertical direction. For example, if there is a heavy sea, the amplitude of the waves can change the distance between the ship and the aerial vehicle by quite a large amount. It is important that an accurate prediction of the movements have been made before landing so that the right occasion for landing can be chosen. It is further important that landing can be performed on a predetermined position, at a safe distance from surface structures and the edge of the platform. In the case of an unmanned aerial vehicle UAV, landing can be performed close to an automatic locking device, securing the vehicle to the platform.
U.S. Pat. No. 4,995,722 discloses a helicopter that incorporates means for emitting a conical light beam, and a landing platform on a ship that incorporates photosensitive cells that detect the conical light beam and calculates the attitude of the helicopter. Landing data are displayed based on the calculated attitude, on a panel which the pilot can see in order to land.
A problem with the system according to U.S. Pat. No. 4,995,722 is that it is dependent on sensitive devices placed on the landing platform, such as photosensitive cells or mirrors. The system is further dependant on additional sensitive devices for transmitting landing data wireless from the landing platform to the helicopter in real time.
WO 2008/061307 discloses a method, which has application in the landing of a helicopter by determining the attitude of a remote surface, wherein a laser transceiver is arranged to direct and detect a laser beam by the use of a mirror, which is used to rotate the emitted laser beam in order to trace out a conical surface. The base of the conical surface describes an oval shape on the landing area, and a number of samples are taken at different rotational position. After a complete scan each 3D-point is stored into a buffer in a memory and passed to a software subroutine which calculates the plane of best fit to the stored points. Thereby the future positions of the remote surface can be predicted. This is used in situations such as when the landing platform is a moving deck of a ship. The method further implements a light emitting beacon in combination with a visual tracking sensor to find a predetermined target position on the landing platform.
A problem with the method according to WO 2008/061307 is that if one transmitter for some reason is disabled the whole system is more or less knocked-out. Furthermore, if the helicopter is too high up in the air there is a risk that the tracking cone is too large and misses the landing platform completely. Additional sensitive devices are also required when implementing an emitting beacon together with a visual tracking sensor to find a target position. These additional sensitive devices risk being affected by disturbance from external light sources, such as lanterns, search lights, stars or reflections from the sea surface.
To sum up, there are known methods in the art for measuring inclination and predetermined positions of a remote landing platform. The problems with the systems according to prior art is that they are vulnerable to malfunctions and inflexible to disruptions.
Thus there is a need of a less vulnerable and more flexible arrangement in order to be able to center aerial vehicles above a predetermined area of a surface, such as e.g. a predetermined area on a moving platform of a ship.